Cadmium Zinc Telluride detectors for a next-generation hard X-ray telescope
Introduction
Cadmium Zinc Telluride (CZT) detectors are an attractive detector technology for hard X-ray astronomy as they offer excellent spatial resolutions, good energy resolutions, and, compared to Si and Ge detectors, much larger photoelectric effect cross sections at hard X-ray energies. A CZT imager may be used on a next-generation telescope succeeding the space-based hard X-ray telescope NuSTAR [1]. The high stoppping power and excellent energy resolution of the NuSTAR CZT detectors enabled it to image the Cassiopeia A (Cas A) supernova remnant in the 67.9 keV and 78.4 keV line emissions from the radioactve isotope Ti [2], [3], [4]. Contrary to the soft X-ray lines detected previously, the nuclear Ti emission directly tracks the yield of nuclear material independent of the temperature and density of the ejecta [2], [3], [4].
The recently developed monocrystalline silicon X-ray mirrors [5] or electro-formed-nickel replicated (ENR) X-ray optics [6] promise angular resolutions with Half Power Diameters (HPD) of between a few arcseconds and 15 arcseconds – even at hard X-ray energies. The proposed HEX-P [7] and BEST [8] observatories seek to capitalize on this technology, as the point source sensitivity scales linearly with the angular resolution. Nyquist sampling the images provided by the improved X-ray mirrors requires detectors with excellent spatial resolutions. Our group is thus leading the development of new small-pixel CZT detectors with center-to-center pitch of 150 microns and hexagonal pixels, improving by a factor of four over NuSTAR’s CZT detectors (605-micron pixel pitch).
This paper discusses the simulations performed for the design of the third-generation Hyperspectral Energy-resolving X-ray Imaging Detector [HEXID3 [9], [10]], which features hexagonal pixels at a next-neighbor pitch of 150 m and uses a low noise front end design achieving a projected readout noise of 14 electrons Root Mean Square (RMS). The advantage of using hexagonal over square pixels is that all the nearest neighbors of any given pixel are equivalent; in square pixels, some immediate neighbors are closer than others. Another similar ASIC for hybridization with pixelated CZT detectors is the High Energy X-ray Imaging Technology (HEXITEC) ASIC developed by Rutherford Appleton Laboratory. The HEXITEC ASIC features 6400 square pixels at a next-neighbor pitch of 250 m with an electronic readout noise of 50 electrons RMS [11], [12], [13].
Our simulations model in detail the interactions of the incident photons, secondary photons and high-energy electrons generated in the CZT, and the ionization losses of the latter. The simulations furthermore model the drift and diffusion of the negative and positive charge carriers through the CZT, including the effects of mutual repulsion of charge carriers of equal polarity. This detailed treatment allows us to predict the properties of the signals, including the pixel multiplicity, and the dependence of the pixel signals on where in the detector the free charge carriers are generated. Earlier discussions of CZT detector simulations can be found in [13], [14], [15], [16], [17]. Compared to the earlier study of small pixel detectors of [13], the shape of our charge clouds evolve owing to charge carrier repulsion and diffusion as the clouds drift inside the detector. Furthermore, we extend the study from a pixel pitch of 250 m to smaller 150 m pixels.
The rest of the paper is organized as follows. After describing the detector simulation methodology in Section 2, we present the results of the simulations in Section 3. Our studies show that the 1mm thick detectors have a limited energy range over which they give excellent performance. We discuss the results and implications for the camera of a NuSTAR follow-up mission in Section 4.
Section snippets
Simulations of the CZT/ASIC Hybrid Detectors
X-rays impinging on a CZT detector interact via photoelectric, scattering, and pair production interactions. Photoelectric interactions dominate up to primary photon energies of at which Compton scattering becomes dominant (assuming 40% Cd, 10% Zn and 50% Te). The photo-electron of a photoelectric effect interaction loses most of its energy to ionization. The ionization promotes electrons to the conduction band, generating clouds of electrons and holes. Applying a bias across the
Results
In this section, we discuss methods to reconstruct the energy of the incident photon, and the location of the primary interaction. We will first discuss the results obtained in the absence of readout noise, and the show how they change as we add the noise expected for the HEXID ASIC.
Discussion
In this paper, we present simulations of 1 mm thick CZT detectors with hexagonal pixels at an extremely small pixel pitch of 150 m. The detector simulations account for the spatially distributed generation of free charge carriers in the detector, and the drift and diffusion of the charge of both polarities. The simulations furthermore account for the anticipated charge resolution of the HEXID3 ASIC. We have shown that the sum of the signals of the brightest pixel and the adjacent pixels and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Grzegorz Deptuch, Gabriella Carini, and Shaorui Li for their work on the HEXID ASIC, as well as the McDonnell Center for the Space Sciences at Washington University in St. Louis for its support. We thank Richard Bose and Andrew West for designing a HEXID readout system and HEXID photomasks. HK acknowledges NASA support under grants 80NSSC18K0264 and NNX16AC42G.
References (53)
- et al.
Pd-hexid1: A low-power, low-noise pixel readout asic for pixelated-scintillator-based X-ray detectors
Proceedings of the 2018 IEEE Nuclear Science Symposium and Medical Imaging Conference Proceedings (NSS/MIC)
(2018) - et al.
Simulation of charge collection processes in semiconductor CdZnTe -ray detectors
Nucl. Instrum. Methods Phys. Res. Section A: Accel. Spectrom. Detect. Assoc. Equip.
(2009) - et al.
Detector performance and defect densities in CdZnTe after two-step annealing
Nucl. Instrum. Methods Phys. Res. A
(2019) - et al.
Growth and characterization of detector-grade CdZnTeSe by horizontal Bridgman technique
SPIE proceedings
(2019) - et al.
Effects of excess Te on flux inclusion formation in the growth of cadmium zinc telluride when forced melt convection is applied
J. Cryst. Growth
(2020) Currents to conductors induced by a moving point charge
J. Appl. Phys.
(1938)- et al.
Charge transport in arrays of semiconductor gamma-ray detectors
Phys. Rev. Lett.
(1995) - et al.
Studying spatial resolution of CZT detectors using sub-pixel positioning for spect
IEEE Trans. Nucl. Sci.
(2014) - et al.
Subpixel resolution in CdTe Timepix3 pixel detectors
J. Synchrotron Radiat.
(2018) - et al.
Characterization of the H3D ASIC Readout System and 6.0 cm 3-D Position Sensitive CdZnTe Detectors
IEEE Trans. Nucl. Sci.
(2012)
Signal modeling of charge sharing effect in simple pixelated CdZnTe detector
J. Korean Phys. Soc.
Development of large-volume high-performance monolithic CZT radiation detector
Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XX
The Nuclear Spectroscopic Telescope Array (NuSTAR) high-energy X-Ray mission
Astrophys. J.
Asymmetries in core-collapse supernovae from maps of radioactive Ti in CassiopeiaA
Nature
44ti gamma-ray emission lines from sn1987a reveal an asymmetric explosion
Science
The distribution of radioactive Ti in Cassiopeia A
Astrophys. J.
Astronomical x-ray optics using mono-crystalline silicon: high resolution, light weight, and low cost
Superhero: design of a new hard-X-ray focusing telescope
Proceedings of the 2015 IEEE Aerospace Conference
Optical instrument design of the high-energy x-ray probe (HEX-P)
SPIE Proceedings
Hexid2: A low-power, low-noise pixel readout asic for hyperspectral energy-resolving X-ray imaging detectors
Proceedings of the 2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC)
HEXITEC: a next generation hard X-ray detector for solar observations
Proceedings of the AAS/Solar Physics Division Abstracts 47
The HEXITEC hard X-ray pixelated CdTe imager for fast solar observations
Modeling and measuring charge sharing in hard X-ray imagers using HEXITEC CdTe detectors
Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series
Spectral calibration and modeling of the NuSTAR CdZnTe pixel detectors
Simulation model for evaluating energy-resolving photon-counting CT detectors based on generalized linear-systems framework
SPIE Proceedings
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Modeling charge cloud dynamics in cross strip semiconductor detectors
2023, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentVertical gradient freeze growth of detector grade CdZnTeSe single crystals
2022, Journal of Crystal GrowthCitation Excerpt :As a wide bandgap and high-Z material with high material density of 5.8 g/cm3, CZTS acts as an excellent gamma photon detector, and due to the superior electron transport properties, it has also demonstrated very high energy resolution for gamma rays in a wide energy range [1,10,17]. Hence, CZTS is a prospective alternative to CZT detectors, which at present is a forerunner room-temperature gamma detectors for medical imaging, space astronomy, homeland security, high-energy physics, and environmental monitoring [18,19,20,21,22]. To achieve detector-grade CZTS crystals, different methods like vertical Bridgman method (VBM), travelling heater method (THM), and horizontal Bridgman method (HBM) have been adopted in the recent years [17,23,24].
Development of small pixel CZT detectors for future hard X-ray missions
2023, Proceedings of SPIE - The International Society for Optical Engineering